Cell Lineage and Segmentation in Development

Abstract

Studies of developmental cell lineage — that is of the fate of individual cells, or blastomeres, that arise in the early embryo — were begun in the 1870’s, in the context of the controversy then raging about Ernst Haeckel’s “biogenetic” law. The biogenetic law seemed to imply that cells of the metazoan blastula recapitulate the non-differentiated tissues of a remote sponge-like ancestor. Only after gastrulation would the germ layers — ectoderm, mesoderm, endoderm - be destined to form the highly differentiated tissues characteristic of more recent metazoan ancestors. This implication was tested by the founder of American experimental embryology, Charles O. WHITMAN (1887) [1]. By observing the cleavage pattern of early leech embryos — which is also the main experimental material of this brief review article — Whitman traced the fate of individual cells from the uncleaved egg to the germ-layer stage and concluded that, contrary to the implication of the biogenetic law, a characteristic postembryonic fate can be assigned to each identified blastomere and to the clone of its descendant cells. These findings suggested, moreover, that the differentiated properties that characterize a given cell of the mature animal are somehow determined by its genealogical line of descent from the egg.